Most of the time, if you open a fillable pdf on Windows w/ Adobe Reader, it warns you that you cannot save the data you entered. However, when opening the same file with Preview on Mac OS you are able to save. This seems to be a choice. Can someone enlighten me?

Saving a filled out form to the hard
drive or emailing it, from a PDF
displayed in Reader requires Reader
Extensions. It doesn't matter what
Acrobat version you are working with.
This has always been true.

You can apply Reader Extensions (also
called Rights) to a PDF with Acrobat 8
professional. Look on the Advanced
menu item.